Power converters can be used in an uninterruptible power supply (UPS) system, of conditioning and safeguarding electric supplies to sensitive or critical equipment. A common use for UPS systems is in data centers. To increase capacity and security of supply, UPS units can be connected in parallel, so that a current supplied to a load is the sum of currents supplied by a number of UPS units. Known UPS units adapted for such parallel applications comprise special features to ensure that “exchange current”, representing an imbalance between an output current of the converter in question and output currents of the parallel converters, is sensed and controlled to be zero. In this way, each UPS automatically adjusts its operation so as to supply an equal share of the total load current. The special features may comprise additional sensing circuits to measure exchange in each unit, and programming of a controller to respond to the sensed exchange current, in addition to other parameters and sensed values.
UPS units are available in a range of sizes, each having for example a nominal power rating such as 100 kVA, 300 kVA, 500 kVA etc. For increased flexibility in the provision of UPS systems, it would be advantageous to be able to connect units with different power ratings in parallel. In such a case, however, the known controller will simply force each unit to take an equal share of the load current. Weaker units will be over-loaded, while stronger units are under-utilized.